This invention relates in general to an improved shadow-mask-type color CRT (cathode ray tube) characterized by having suppressed electron flooding of the screen by overscanned electrons. It is of course desirable that the picture produced by a television receiver completely fill the visible screen area of the faceplate. To assure this, and thus to prevent any possibility that the ragged edge of the television raster may be seen by the viewer, it is standard practice to "overscan" the viewing area of the screen. That is, the electron beam deflection system is designed and set up such that it makes a picture typically about 7% larger than the visible area of the screen. The outer edge of the picture is wasted, however this is considered to be necessary to assure that the edge of the raster will not be seen.
As a consequence of this deliberate "overscanning" of the visible screen area, at the ends of each excursion of the electron beams, the beams play on exposed areas of the shadow mask. (As is well known, all modern color television tubes utilize a shadow mask to assure that the three electron beams associated with red, green and blue picture information excite only the red-light-emitting, green-light-emitting and blue-light-emitting phosphors, respectively).
However, if provisions are not made, overscanned electrons may reflect off surfaces of the mask (much in the manner in which light reflects off a mirror) and "flood" the screen near its perimeter. Since the scattered electrons lose their color-associated trajectories upon reflection, they impinge upon the screen randomly, producing a visible white light under certain viewing and picture conditions which may degrade the television picture. The reflected electrons also do not represent useful picture brightness intelligence at the point of impingement on the screen. This visible electron flooding of the peripherial area of the screen is, in its visual appearance, not unlike the lights of a city over the horizon.
Conventional shadow mask assemblies include a thin perforate mask member and a heavy rigid frame which is used to give mechanical support to the mask member. In order to prevent overscanned electrons from reflecting off the inside surface of the frame and the rearwardly turned skirt of the mask member, it is conventional to form an inward flange on the back side of the frame which serves as a shield to block passage of overscanned electrons.
It is also known in applications where the shadow mask has no frame, such as disclosed in U.S. Pat. No. 3,912,963, to configure the shadow mask in such a way as to prevent overscanned electrons from being reflected off exposed surfaces onto the visible area of the screen. It is also known from the '963 patent to form ribs in the skirt of a shadow mask assembly for strengthening purposes.
It is also known, for reasons completely unrelated to suppression of visible electron flooding of the screen, to partially pre-etch a pattern of recesses on the outside surface of a shadow mask member in the heel radius region. This is done to minimize stretching and distortion of the shadow mask apertures when the shadow mask member is mechanically formed. See U.S. Pat. No. 3,809,945. Also unrelated--some photographic cameras have baffles to minimize stray ambient light reaching the film plane.
The invention has general applicability to shadow masks of various configurations and types, but perhaps has particular applicability to a CRT having a shadow mask characterized by the lack of a frame and the presence of a rearwardly directed skirt. With such a frameless mask, overscanned electrons are free to reflect off the exposed inner surface of the rearwardly turned skirt and randomly flood the screen through the shadow mask perforations.